Air-conditioning, especially in tropical climates, is dictated by the stringent requirements of cooling and dehumidification. The HVAC engineer must seek an optimal design, balancing energy consumption and cost, thermal comfort, and indoor air quality (IAQ), the latter criterion being a function of how well the building is ventilated. Although health and thermal comfort issues can be usually addressed by attempting to eliminate the various sources of indoor pollutants, it is almost impossible to totally eliminate them. It is thus inevitable that ventilation with fresh air should play an important role in the eventual quality of the air in the indoor environment. The high energy-penalty of cooling and dehumidification associated with ventilation in the tropical context is apparent from the large enthalpy difference that exists between the outside air and the indoor air conditions.
Inadequate ventilation almost invariably leads to a deterioration of the indoor air quality due to a build-up of indoor pollutants such as carbon dioxide. Adequate ventilation with poor dehumidifying performance of the cooling coil would be even worse as it is likely to result in elevated humidity levels in the air distribution systems and the occupied zones. This phenomenon could be considered in the context of humidity as an “indoor” pollutant, which propagates the origin and sustenance of microbial contamination. The issue of high indoor humidity levels in tropical buildings originates from the high ambient humidity levels in the ventilation air (fresh air) and the need to provide adequate ventilation to avoid “Sick Building Syndrome”.
The basic requirements of any air-conditioning system are to provide comfortable temperature and humidity conditions for the occupants of a building. In a steady-state environment, this is a relatively simple task. However, in a dynamic indoor environment, where cooling and dehumidifying requirements fluctuate with changes in weather and occupancy, designing an energy-efficient air-conditioning is anything but a simple task. Optimizing the design of an air-conditioning system is an even more arduous task in hot and humid climates, where the fresh air contains extremely high humidity levels at all times of the year.
In typical cooling and dehumidifying coil designs, an air stream having an intake temperature and humidity flows over the tubes and fins of a coil while chilled water flows through the tubes of the coil. The chilled water enters the coil at a supply temperature and leaves at a return temperature. The psychrometric performance of the cooling coil determines the cooling and dehumidifying of the air flowing past the coil. In the event of two different air streams being conditioned, two separate coils would typically be employed with either a parallel or a sequential feed of chilled water.